A conventional CMOS imager pixel cell comprises a photoelectric conversion device such as a photodiode for converting light into an electrical charge, a floating diffusion node for receiving the electrical charges, a reset transistor for resetting the floating diffusion node to a predetermined charge state, a source follower transistor for receiving and amplifying a voltage on the floating diffusion node and a row select transistor for gating the source follower transistor output onto a column line. In some pixel cells, a transfer transistor is used to gate charges from the photoelectric conversion device to the floating diffusion node. Each pixel cell also has an associated connection from the floating diffusion node to the source follower transistor. There are also typically other connections to, for example, the reset transistor, row select transistor and transfer transistor, if employed. These connections, comprising metal lines, pass through layers of insulator material that are light-transmissive and run along the surfaces of the insulator layers, forming one or more metallization layers.
A passivation layer is also typically deposited over the final metallization layer by known conventional methods. This deposition may cause a “bread-loafing” effect above the metal lines as the top surface of the passivation material rises up and over the upper metal lines as well as weak areas in between closely spaced conductive lines. Accordingly, a non-uniform passivation layer is produced, which may cause a non-uniform floor for a subsequent filter array coating, which may in turn lead to stress-induced striations, poor color performance and low predictability of the overall image captured by the pixel cell array. The passivation layer can include a deposited oxide, a deposited nitride, or a composite stack of oxide/nitride layers.
In the case of composite stack oxide/nitride layers, the “bread-loafing” effect in the oxide passivation layer over the metal lines also results in very thin nitride coverage over the metal lines at certain metal spacings. Where nitride coverage is thin, or weak, mobile contaminants such as sodium (Na), potassium (K) and copper (Cu) can penetrate into the silicon device region and cause loss of pixel functionality. In some cases, these weak regions, directly adjacent to regions with thick nitride coverage have particularly high stress and may result in nitride layer cracking. This is of particular concern for imaging devices as the color filter array materials typically contain Na and Cu contaminant species. Therefore, a more uniform, flat oxide passivation layer over the final layer of metal lines will allow for a much thinner nitride passivation deposition, reducing stress in the structure of the passivation layer of the pixel cell.